For those of us who are holding out hope that they aren't actual cousins:
The original text for the bubble is:
「私たち従姉妹でしょ
」
Which, when literally translated, reads:
"We are cousins, right?" (The scanlator's use of ". . . aren't we?" is absolutely appropriate here.)
The key word here is
従姉妹 (いとこ), which is the most cut-and-dried word for blood-related cousins that the language has.
I'd like to emphasize that the translation presented by our dear scanlator (for this particular bubble) is absolutely correct in every conceivable way. Whoever suggested that it may be erroneous hasn't done their homework!
As a small aside, consanguineous (cool word I just learned) marriage is indeed legal in Japan.